Patients with rapid cycling bipolar experience at least four episodes of affective illness (depression, hypomania, and/or mania) in a year. They suffer significant morbidity and are frequently resistant to conventional treatments. These patients experience dramatic shifts in their sleep-wake cycle as their mood state changes. Furthermore, results from another study (Z01 MH 02614-01 CP) show consistent differences in their circadian rhythms in the depressed and hypomanic states. Since treatment with bright lights (phototherapy) has been shown to shift circadian rhythms in humans, we are conducting a controlled trial of phototherapy in patients with rapid cycling bipolar disorder to see if bright light can be used to shift their circadian rhythms and alleviate the depressive symptoms that they experience. Throughout the study, patients remain on a stable regimen of psychotropic medication. They are followed through several depressive episodes in order to establish the baseline pattern of their illness. two consecutive depressive episodes are then treated with phototherapy (one hour of 10,000 lux). Patients are randomly assigned to treatment in either the morning or evening, and then crossed over to the other condition during the second episode. Mood is monitored through daily self-ratings and weekly observer ratings. Ten days after beginning each of the phototherapy conditions, patients are admitted to the hospital for an evening in order to undergo a DLMO (dim light melatonin onset) procedure. During this procedure, patients are kept in a dimly lit room while blood samples rare withdrawn to measure the onset of melatonin secretion. The purpose of the DLMO is to determine whether the phototherapy has, as predicted, shifted the patients' circadian rhythms.